Calcined gypsum has hydraulic properties (setting on reacting with water) and is thereby used as a model material in various fields. In particular, it is inevitable as a denture model material for preparation of various dental prostheses that are fitted into the oral cavity, such as a full denture, a denture, a crown, a bridge, an inlay, and an occluding frame.
A denture model is generally prepared by making a negative mold called an impression using an impression material, and pouring a slurry of calcined gypsum in water into the mold, followed by setting. Known impression materials include rubbers, e.g., silicone rubbers, polyether rubbers, and polysulfite rubbers; and aqueous colloids, e.g., agar and alginates.
The denture model materials are required to have (1) dimensional precision, (2) mechanical strength, (3) capability of forming a smooth surface in contact with various impression materials, (4) easy handling, (5) stability with time, and (6) easy processing, and (7) to undergo no appreciable foaming.
In order to improve dimensional precision, it is generally necessary to control expansion of gypsum on setting by addition of an expansion inhibitor, such as sulfates, tartrates, and oxalates as disclosed, e.g., in JP-A-50-161492 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In order to obtain a high-strength, high-hardness and smooth-surfaced gypsum model, it is known to add to .alpha.-type calcined gypsum a metal sulfate and a melamine-formaldehyde resin as disclosed, e.g., in JP-A-62-270451.
While aqueous colloidal impression materials, such as agar and alginates, are disadvantageous in that gypsum models patterned thereon have poor surface smoothness, a composite material composed of agar and an alginate have recently been developed, by which both merits of agar and easy handling and good economy of alginates can be taken advantage of, and has now found frequent clinical use.
However, agar-based impression materials, though excellent in affinity for oral tissues, still involve several problems that need to be addressed in terms of compatibility with gypsum and dimensional stability. That is, gypsum models received from an agar impression sometimes suffer from considerable surface roughness depending on the combination of an agar impression material and a gypsum model material.
For example, JP-A-62-270451 supra states that a gypsum model having a smooth surface can be obtained even when combined with an aqueous colloidal impression material but, in fact, cases are met in which the resulting gypsum model suffers from considerable surface roughening when combined with some impression materials other than specific ones, for example, when combined with an agar impression material. Therefore, a complete solution has not yet been reached. It has thus been demanded to develop a gypsum composition which provides a gypsum model having a smooth surface with dimensional precision without any limitation of an impression material to be combined with.